DE2358688B2 - Process for increasing the service life of organic extractants - Google Patents

Description

The invention relates to a process for increasing the lifespan of organophosphoric acid ester and Extractants containing hydrocarbons, which are used for the reprocessing of highly spent Nuclear fuel and / or breeding material can be used by removing the chemical and / or Radiolytic decomposition creates contaminants and interfering compounds of these substances with radionuclides.

Interfering substances or interfering compounds are understood here that are similar to the decomposition products, for example of tributyl phosphate (TBP), dibutyl phosphate (HDBP) and monobutyl phosphate (H ₂ MBP). or the decomposition products of other organophosphoric acids, but also compounds of previously unknown structure, accumulate in the extraction agents as such or in the form of complexes with actinides and / or fission product nuclides or corrosion products and reduce the extraction efficiency.

For example, the radiolytic or chemically formed decomposition products have an effect in the system TBP-alkane (e.g. 20% by volume TBP-80% by volume dodecane) and others. a retention of fission products (such as Zr-95) in the organic phase and thus lower the decontamination factors of the organic product streams of remanufacturing processes. If these interfering compounds accumulate too much in the Extractant cannot be reused and must be used as organic waste liquid be discarded.

The decomposition products formed in the _{extractant mixture of TBP and aliphatic hydrocarbons (e.g. C10H22 to CuH 2} S), which are mainly used today for the reprocessing of nuclear fuel and / or breeding material _{, can only be partially removed (especially H 2} MBP and HDBP). remove from the organic phase by washing with sodium carbonate solution.

A number of chemical compounds of previously unknown structure are removed by the alkali wash however, only insufficiently separated, so that their concentration increases with repeated use of the Solvent increased in it. This results in undesirable effects, such as poorer separation of organic and inorganic phase. The separation of the valuable substances uranium and plutonium from the fission products is considerably worsened by the extraction of fission products into the organic phase. For longer Twisting of the extractant thereby also becomes the extractant of an additional one Exposed to radiation dose which leads to further decomposition phenomena. Through the processes described becomes the usability, d. H. the life of the extractant, limited; it finally has to go through a fresh batch must be replaced. Does this need occur too often? so falls a considerable Amount of radioactive, organic waste liquid which then increases the profitability of the extraction process challenged by the increased cost of waste disposal.

For cleaning the extraction agent or the solvent (e.g. aliphatic hydrocarbons) of the interfering substances mentioned, no method has been available so far that is effective and is economical at the same time.

By adsorption on solids such as aluminum oxide or activated carbon, about 50-60% of the disturbing Connections are removed (C. A. Blake, Jr .; W. D a ν i s, Jr .; and J. M. S c h m i 11: Nuclear Science and Engineering 17, pp. 626-637 [1963]), another 20-25%

jo only through the use of relatively large amounts of the adsorber. Of other cleaning methods, washing with an alkaline permanganate solution is particularly effective. The precipitation of solid MnO ₂ during this treatment is an undesirable phenomenon due to the associated disruption in the phase separation. Alkanolamines show good effectiveness as washing solutions. However, this process is of little economic interest because of the high material costs and the losses of tributyl phosphate that occur in the process. Of all cleaning processes, distillation in the form of molecular distillation or flash evaporation is still an option. However, distillation is generally known to be an expensive process step.

The invention is therefore based on the object of creating a method that has the disadvantages of the known Process avoids contaminants and disruptive compounds from extraction agents practically completely removed and the usability or lifespan of the extractants increased without loss of effectiveness.

The object is achieved according to the invention by

that the neutral or acidic extractants are brought into contact after their use with tetravalent lead compounds at temperatures in the range between room temperature and approx. 100 ^° C., all contaminants being converted into an easily removable state, then in a manner known per se treated with an alkali wash to remove the contaminants and recycled for reuse. One or more compounds from the group lead dioxide PbO ₂ , lead tetraacetate Pb (CH ₃ COO) ₄ can be used as tetravalent lead compounds. Advantageously, the extractants are continuously

b5 ierlich over a heatable column with a silica gel-lead dioxide mixture directed.

Through the contact of the contaminants or interfering compounds, the complex-forming properties

have, with tetravalent lead compounds at elevated temperature (for example, in the order of 50 ⁰ C) are formed oxidation products, can easily be removed. A reaction time of less than 20 minutes is sufficient. After subsequent washing with 0.5 M Na ₂ CO ₃ solution in an organic-aqueous volume ratio of 1: 1, more than 90% of the complexing agents are destroyed or removed. The composition of the organic phase, insofar as it is important for process control (ζ. Β. Ι ο TBP content), has not changed as a result of this treatment.

As a measure of the degradation of the solvent, it is common to examine the extraction of zirconium or hafnium by the organic phase. In these tests, the sample is shaken with an equal volume of 3-M HNO ₃ , which contains radioactive Zr ⁹⁵ or Hf »or Hf ¹⁸¹ , for 10 minutes and the zirconium or Hf quantities extracted into the organic phase are determined by the characteristic y- Radiation determined For comparison, the test is carried out on a non-irradiated sample in the same way.

The invention is illustrated below with the aid of some examples, which, however, do not restrict the Represent invention:

example 1

To 10 ml of irradiated vol 20 ° / o TBP-80 vol .-% alkane (Cn-Cio) mixture (radiation dose 40 Wh / 1, ⁶⁰ Co source, equilibrated with 0.5M HNO3), 100 mg Pb (CH ₃ COO) ₄ given and ^{stirred at 50 0} C for 20 minutes. The solution was centrifuged to separate the precipitated PbÜ2 and then _{washed with 1 M Na 2} CO ₃ solution. An extraction test with i «18iHf in 3 M ENT ₃ showed the effectiveness of the treatment:

Extraction of org. phase

before treatment
Extraction of org. phase

after treatment
Extraction of a

unirradiated sample

19,000 pulses / minml
700 pulses / min ml
200 pulses / min ml

Example 2

washed and tested for extractability with i ».miHf in 3 M HNO ₃

Extraction of the sample

before treatment
Extraction of the sample

after treatment
Extraction of a

unirradiated sample

40

45

To 10 ml of irradiated 20 vol .-% TBP 80 vol .-% Aikan (Cio-C | ₃₎ mixture (radiation dose 40 Wh / 1, ⁶⁰ Co source, equilibrated with 0.5 M HNO _3), 100 mg PbO ₂ given and treated at 50 ° C for 20 minutes. The solution was centrifuged with 1 M Na2CO ₃ solution 19,000Imp./minml
1 450 pulses / min ml
200 ImpVmin ml

Example 3

A heatable column was filled with a mixture of 90% by weight silica gel and 10% by weight PbO ₂ (column volume 15 ml, column content in total 0.75 g PbO ₂ ). The column was heated to 50 ^° C. A solution of 20% by volume of TBP-80% by volume of alkane (Cio-Ci ₃ ) was irradiated with a total dose of 40 Wh / l of ⁶⁰ Co radiation and then _{washed with 1 M Na 2} CO ₃ solution. After equilibration with 0.5 M HNO ₃ , the solution was allowed to flow _{through the PbO 2} -SiO _{2 column.} Samples were taken from the leaked solution and the extraction tests carried out. Before treatment with the column, the solution had 92,000 pulses / min · ml ¹⁷⁵ · "" Hf extracted _{from 3 M HNO 3.} The results are shown in the table below. The extraction capacity before the treatment was set at 100%. It can be seen from the table that even with a throughflow of the amount of solvent to be purified which corresponds to ten times the volume of acid, the liquid flowing out of the column has an extraction capacity of more than 90%.

Solvent purification using a 10% column
PbOi - 90% S1O2
Columnar vol. 15 ml

Enforced
solvent

Me ⁴ + (Hf) extraction

(Imp./min ml)

Solvent leaking from the column still has (%) extraction capacity

50

92,000 100 387 0.43 838 0.93 850 0.95 2,476 2.75 8 299 9.2 5,464 6.1 6 496 7.2

Claims (1)

Claim: Process for increasing the life of organophosphoric acid esters and hydrocarbons containing extractants, which are used for the reprocessing of highly burned nuclear fuels and / or breeding materials, by removing the interfering substances and interfering compounds of these substances with radionuclides caused by chemical and / or radiolytic decomposition that the neutral or acidic extractants are brought into contact after their use with tetravalent lead compounds at temperatures in the range between room temperature and approx. 100 ^° C., all contaminants being converted into an easily removable state, then in a manner known per se treated with an alkali wash to remove the contaminants and recycled for reuse.